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FEC on over North America



The FEC (forward error correction) mode is enabled on AO-40 this 
morning. I'm sitting on my patio in San Diego with a laptop and R-7000 
receiver connected to a G3RUH dish/feed and UEK-3000 downconverter 
propped up on a patio chair.

The FEC telemetry is absolutely solid even though I'm getting a fair bit 
of random impulse QRM (from nearby cordless phones?) I can offprint the 
dish until the beacon is barely audible to the ear and it still works.

Here is a sample debug output from my program:

Carrier        Clock          symbol time     Es/No
1560           399.610        1               18.0
1560           399.594        0               18.1
1570           399.610        1               18.8
1580           399.594        0               19.7
1590           399.577        0               21.2
1580           399.594        0               19.1
1590           399.577        0               20.3
1590           399.594        0               20.2
1580           399.577        0               18.8
1600           399.594        0               21.5
1580           399.577        0               18.2
1560           399.594        0               17.4
1570           399.594        0               18.2
1590           399.610        1               21.0
1610           399.594        0               22.8
1610           399.577        0               22.2
1610           399.594        0               21.7
1610           399.577        0               22.7
1600           399.594        0               21.6
1600           399.577        0               21.0
1610           399.561        -1              22.7
1600           399.577        0               20.9
1610           399.561        -1              22.2
1610           399.577        0               22.3
1610           399.594        0               21.3
1600           399.594        0               19.7
1580           399.610        1               18.0
1580           399.594        0               18.1
1580           399.610        1               18.3
1580           399.594        0               18.2
1590           399.577        0               18.9
correlator amp 35.5 dB energy = 87.2 dB scale = 206
VD histogram: 273 543 767 633 230 51 15 2 0 2 4 9 6 19 88 2493
RS byte corrections: 0 0 - Good Frame
channel symbol error corrections: 1 (0.0192%)
Decode attempts: 5987 Partial decodes: 0 Full decodes: 109

(end of excerpt)

It first shows, at one-second intervals, the estimated carrier 
frequency, symbol clock frequency, symbol time offset and Es/No (the 
signal-to-noise ratio per channel symbol). Since the telemetry format 
consists of a conventional (uncoded) frame (which I am currently 
ignoring) followed by a FEC frame, I have to process something like 31 
seconds to get one 13-sec FEC frame.

The five lines at the bottom of the listing show that only one channel 
symbol error had to be corrected. This was done by the Viterbi decoder; 
the Reed-Solomon decoder had nothing to do. If this had been an uncoded 
frame, that single error would have been enough to ruin it. But with 
FEC, I can even retune the receiver in the middle of a frame and still 
easily decode it.

The large "Decode attempts" figure does *not* imply that I've missed 
that many frames. (In fact, I've missed none). Because the frames come 
down asynchronously, I can't be sure where a frame is in time even after 
successfully decoding a previous frame. So I take the very conservative 
approach of attempting a decode whenever the sync correlator produces 
even a small peak. If I'm wrong, all I do is waste a little CPU time. 
But I won't miss any actual frames that way.

My software is up on my web page as

http://www.ka9q.net/ao40/ao40prototype-4.1.tar.gz

See also http://www.ka9q.net/ao40/ for more information on the FEC 
format plus several other software packages needed to make this work.

This is *very early prototype* software, for Linux only, that I'm still 
working on daily; the DBPSK demodulator in particular needs a lot of 
work on its search and tracking algorithms. It is far from being a 
turnkey package. That will hopefully come later. But the initial results 
are very good.

73, Phil



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